This invention relates to optical systems. More particularly, the invention is directed to radiant energy beam integration optics for improving beam intensity profile uniformity in the case of various lasers or other radiant energy sources having a nonuniform beam intensity profile characteristic.
For example, ultraviolet (UV) excimer lasers have recently been applied as semiconductor processing tools. Typical applications have included semiconductor annealing, microphotolithography, photodeposition, laser-induced chemical vapor deposition (CVD), gas immersion laser doping (GILDing), micromachining, and several other processes. In nearly all of these applications, laser output beam intensity profile uniformity is of paramount importance. Hereafter the term "beam uniformity" will be employed to refer to beam intensity profile uniformity. Present discharge UV excimer laser technology does not produce laser output beams of adequate uniformity while maintaining required laser output energy.
Currently, most of the work invested toward improving UV excimer laser output beam uniformity has concerned the laser configuration itself. Optical resonator design, electrode profiling techniques, and improvement of discharge preionization uniformity have increased the laser output beam uniformity significantly. By using available technology, it is possible to construct excimer lasers with relatively uniform output beam profiles. However, typical laser output beam uniformity of even +5 percent or so may be only marginally suitable for some illumination applications in which the laser output beam must be reduced to typical semiconductor die sizes in the range of 0.5 to 2.0 cm.sup.2. In addition, few if any commercial UV excimer lasers maintain this level of uniformity over enough area of their output beams to ensure sufficient energy density. Further complicating this problem is the presence of occasional and essentially unpredictable changes in laser output beam uniformity on a shot-to-shot basis. Also, as semiconductor structures and device tolerances become smaller, the requirements for laser output beam uniformity become more severe. Therefore, the future development of semiconductor processing techniques using UV excimer lasers will require increasingly uniform laser output beams.
In contrast to optimizing the configuration of the laser itself, the present invention relates to improving beam uniformity based on optical techniques which act on the laser output beam. Specifically, the invention is directed to optical beam integration techniques.
Optical integrators have been incorporated into various types of illumination systems for many years. In most of these optical integrators, the homogenization of the input beam occurs in one of two ways. Optical integration techniques typically involve either some kind of randomization of the laser output beam (in phase or amplitude) or optical integration performed by the overlapping of numerous beam segments. The input beam can either be "scrambled" by a diffuser; a set of lenses with partially overlapping outputs (Oriel Corporation, 15 Market Street, Stamford, CT 06902, product model 6567-1, for example); random phase shift masks (Y. Kato and K. Mima, Appl. Physics B29, 186 (1982)) or echelons (R.H. Lehmberg and S.P. Obenschain, Optics Comm. 46, 27 (1983)); or by multiple scatterings in a tube much like a kaleidoscope (R.E. Grojean, D. Feldman, and J.F. Roach, Rev. Sci. Inst. 51, 375 (1980)). Alternatively, the input beam can be broken apart into segments and these segments then imaged on top of one another to average out fluctuations in beam intensity. FIGS. 1 and 2 illustrate typical optical integrator configurations for each of these classes.
On the one hand, FIG. 1 shows one example of a known optical integrator configuration in which the input beam is converging to a confocal spherical lens pair, each lens unit comprising several small spherical lenses mounted in a regular array. A portion of the outputs of these small lenses overlaps. This "scrambles" the now diverging beam in the near field. Since only a portion of the outputs of these small lenses overlaps, however, a substantial amount of the radiant energy that is input is wasted. An optical correcting lens (shown in FIG. 1) may be used to then obtain a somewhat collimated output. This configuration, however, produces images of the lens array in the far field, and thus the working distance is limited. In addition, if the scrambling is incomplete, input beam "hot spots" and other severe nonuniformities are not effectively removed. A focusing lens alone is not used and there is no adjustability of the size of the image. Finally, scrambling may not be useful in applications in which the coherence of the original laser output beam is an important factor for imaging. The incorporation of this configuration or any other refractive optical integrator into a laser is not known.
On the other hand, FIG. 2 shows a known configuration currently used for optical integration by overlapping many individual beam segments. For the sake of clarity, an input beam segment (S) diverges from one mirror in a cone of useful aperture (A) and central ray (C) to illuminate a focusing lens. In this configuration, a single array of spherical mirrors is illuminated by the input beam. The reflected light from each of these mirrors then expands, and if there is proper alignment, the reflected light will be collected by the focusing lens. While this configuration performs rudimentary optical integration, the incidence angle required for the mirror array limits the size of the input beam, and a significant fraction of the incident light may be lost beyond the diameter of the focusing lens. This also severely limits the output spot size available in typical configurations. In addition, off-axis beam displacements created by this configuration can provide undesirable complications in alignment and use. This configuration has been incorporated into point source laser-based holography systems in which input beam intensity profile uniformity is not as critical as in the case of semiconductor processing.
Unfortunately, the beam uniformity produced by known optical integrators would not be satisfactory for many applications, such as in the field of semiconductor processing. Furthermore, known optical integrators are configured so that the spot in the work plane has a fixed size and is also a fixed shape.